Bonding in aryl halides

Aryl halides are compounds in which a halogen substituent is attached directly to an aromatic ring. Representative aryl halides include 

The carbon-halogen bonds of aryl halides are both shorter and stronger than the carbon-halogen bonds of alkyl halides, and in this respect as well as in their chemical behavior, they resemble vinyl halides more than alkyl halides. A hybridization effect 

TABLE 23.1 Carbon-Hydrogen and Carbon-Chlorine Bond Dissociation Energies of Selected Compounds  

PROBLEM 23.1 Consider all the isomers of C7H7CI containing a benzene ring and write the structure of the one that has the weakest carbon-chlorine bond as measured by its bond dissociation energy. 

The strength of their carbon-halogen bonds causes aryl halides to react very slowly in reactions in which carbon-halogen bond cleavage is rate-determining, as in nucleophilic substitution, for example. Later in this chapter we will see examples of such reactions that do take place at reasonable rates but proceed by mechanisms distinctly different from the classical SnI and Sn2 pathways. 

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Aryl halides can also be reduced by tin hydrides76,77, although these reactions always require initiators because the stronger C—X bonds in aryl halides are less reactive than the C—X bonds in alkyl halides. In fact, a series of meta- and para-substituted bromobenzenes, where X is either meta- or para-CH3O-, C=N, Cl, F, CF3, CH3, Bu-f or 2,6-dichloro, have been reduced by tributyltin deuteride (equation 60). It is worth noting that the more reactive bromide is reduced selectively in the presence of the less reactive chloride and fluoride groups (equation 61). 

HYDROGENOLYSIS OF THE TRIGONAL CARBON-HALOGEN BOND IN ARYL HALIDES... 

Intramolecular dissociative electron transfer in aryl halide radical-anions involves an interaction between the n-aroinatic orbital and the o-type carbon-halogen bond. These orbitals are orthogonal, but bending of the carbon-halogen bond allows the necessary interaction. Qualitatively, the influence of several factors on the bond cleavage rate can be discerned [18],... 

Aryl halides are relatively unreactive toward nucleophilic substitution reactions. This lack of reactivity is due to several factors. Steric hindrance caused by the benzene ring of the aryl halide prevents SN2 reactions. Likewise, phenyl cations are unstable, thus making SN1 reactions impossible. In addition, the carbon-halogen bond is shorter and therefore stronger in aryl halides than in alkyl halides. The carbon-halogen bond is shortened in aryl halides for two reasons. First, the carbon atom in aryl halides is sp2 hybridized instead of sp3 hybridized as in alkyl halides. Second, the carbon-halogen bond has partial double bond characteristics because of resonance. 

Carbon-halogen bond cleavage in aryl halides is believed to involve initial excitation into an upper singlet state that is stable with respect to cleavage, followed by intersystem crossing to an upper triplet state that is either dissociative itself or can cross to an upper dissociative (a, a ) triplet [65-67]. The two-color approach has been used to demonstrate directly that excitation to an upper triplet state results in carbon-halogen cleavage. For example, in 2-bromonaphthalene (81) and 9-bromo-phenanthrene (82) [68], promotion of Tj to an upper triplet by dye laser excitation... 

Substitution of halogen by hydrogen in aryl halides can take place without, or with only partial, reduction of carbon-carbon double bonds, aldehydes, nitriles, and even readily reducible nitro groups - present in the molecule (equations 29-32). 

Now, as we have seen (Sec. 5.2), a double bond is shorter than a single bond joining the same pair of atoms if the carbon-halogen bond in aryl and vinyl halides has double-bond character, it should be shorter than the carbon-halogen bond in alkyl halides. Alternatively, a bond formed by overlap of an sp orbital should be shorter than the corresponding bond involving an sp orbital. 

What about unactivated C-X bonds, such as those found in aryl halides, which also readily undergo OA In Chapter 12 we will see many useful synthetic transformations where OA of an aryl C-X bond is the first step in a catalytic cycle. The SN2 pathway is not available for aryl halides, so another mode of addition must occur. Especially informative studies have been reported on the OA of Ar-X to L2Pd (L = phosphine), which is often involved in the catalytic cycles mentioned above. These investigations94 have shown that OA occurs after dissociation of one of the L ligands, and calculations95 not only corroborate the experimental work, but also suggest that the step involving OA is probably more like the three-centered, concerted step found with C-H and C-C addition. 

Suzuki, Stille, and related couplings. The oxidative addition of the C-B bond in aryl and alkenyl boronic acids to Pd(0) initiates the coupling with aryl halides or... 

Aryl halides are much less reactive than alkyl halides in nucleophilic substitution reactions. The carbon-halogen bonds of aryl halides are too strong, and aryl cations are too high in energy, to permit aryl halides to ionize readily in S l-type processes. Furthermore, as Figure 12.7 shows, the optimal transition-state geometry required for 8 2 processes... 

In aryl halides and vinyl halides the C - X bond is very strong, as the lone pair of electrons on halogen atom gets delocalized which gives the C - X bond, partial double bond character. Moreover, the C-X bond is formed by sp - p orbital overlap. This makes the bond short and strong. Thus, it is very difficult to cleave the C-X bond of aryl and vinyl halides and they do not give white precipitate with silver nitrate solution. 

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